In recent years liquid separation and purification systems utilizing reverse osmosis have been applied in many fields such as the desalination of sea water or other saline waters and in the recovery of valuable materials from waste liquids of various types.
Various semipermeable membranes are now being used in commercial reverse osmosis treatments of aqueous solutions, either for water purification or for concentration of liquid solutions, or both. Such semipermeable membranes include the early Loeb type membranes made of cellulose acetate by processes as described in U.S. Pat. Nos. 3,133,132 and 3,133,137. The Loeb type membranes are asymmetric types and are characterized by a very thin, dense surface layer or skin that is supported upon an integrally attached, thicker supporting layer. However, the cellulose acetate Loeb type membranes are restricted as to use and as to processability, largely because the membranes must be kept wet at all times; their effectiveness as reverse osmosis membranes is lost once the membranes are dried.
These membranes have also exhibited deficiencies such as alkaline or acidic degradation and biological degradations which result in short life. Furthermore, these membranes are not used widely in separation or recovery of valuable materials from liquid mixtures containing organic chemicals, because the membranes have low selectivity for valuable organic materials and poor resistance to the effects of organic solvents.
Other Loeb type membranes which are also used include membranes fabricated from polyamides (please refer to U.S. Pat. No. 3,567,632, for example), polyamide hydrazide, polyamide acid (Japanese Patent Publication No. 50-121, 168), crosslinked polyamide (Japanese Patent Publication No. 52-152,879), polyimidazopyron, polybenzimidazole, polysulfonamide, polybenzimidazolon, polyaryleneoxide, polyvinylmethylether, polyacrylonitril, polyvinylalcohol, polyhydroxethylmethacrylate and polyvinylidinechloride, etc. However, the separation performances and the resistances to chemical degradation of these Loeb type membranes are all inferior to those of the cellulose acetate membranes.
When utilizing the semipermeable membranes in the treatment of brackish water, particularly the treatment of sea water, it is often necessary to treat the feed materials with chlorine or other oxidizing agents so as to guard against bacterial growth which could greatly deter the performance of the membranes as a result of fouling or the like. Therefore, this chemical degradation in the membranes results in too short a useful life, with accompanying too-low salt rejection, which results in an inefficient operation.
In a later development, processes have been provided for preparing an ultra-thin film or skin separately from a porous supporting layer.
Membranes so prepared have become known as composite membranes. In preparing such membranes it is possible to tailor both the ultra-thin film and the porous supporting layer in such a manner that each component possesses the most desirable properties. Processes for preparation of composite membranes are described in U.S. Pat. Nos. 3,744,642 and 3,926,798 and in Publication Board Reports Nos. 234,198 and 248,670 published by the National Technical Information Service, U.S. Department of Commerce.
Generally, these composite membranes have also exhibited deficiencies such as compaction resulting in short life, as well as undesirably low flux or solute rejection, all resulting in inefficient operation.